An important way to identify
the missing quasars with \(\rm 2.2<z<3.0\)  has been suggested by using the infrared K-band excess based on the UKIRT
(UK Infrared Telescope) Infrared Deep Sky Survey (UKIDSS) [1]}, [2]}, [3]}.The UKIDSS project is defined in [4]}. UKIDSS uses the UKIRT Wide Field Camera (WFCAM;
[5]} and a photometric system described in [2]}.
The pipeline processing and science archive are described in
[7]}. Although the \(z\sim \) 2.7 quasars have
optical colors similar to stars, they are usually more luminous in the infrared
K-band. In addition, combining the optical colors in SDSS with the infrared
colors in UKIDSS, it should be more efficient to separate stars from both
lower redshift quasars (\(z<\) 3)[8]} and higher redshift ones
(\(z>\) 6)
[2]} in the color-color diagrams. Recent studies by [10]}
and [11]} have demonstrated that with the SDSS-UKIDSS photometric data we can
efficiently select quasar candidates at \(z<4\)  with the selection criterion in the Y-K versus \(g-z\)  color-color diagrams. The spectroscopic observations carried out
by [12]}, [13]} and [11]} have confirmed the effectiveness of using
SDSS-UKIDSS colors to discover the missing SDSS quasars with \(\rm 2.2<z<3.0\) .
